The now-settled lawsuits brought by thousands of former players hinged on the notion that the NFL ignored decades of research on the degenerative long-term effects of brain trauma, and didn't take enough steps to protect its employees. Here's a helpful (ahem, Pete Prisco), but necessarily incomplete timeline of concussion research and the NFL's response. Some of this relies on the excellent timeline put together by Ta-Nehisi Coates and Malcolm Burnley.

CBS Sports football columnist Pete Prisco has some thoughts about the NFL's concussion…
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1933: The NCAA's medical handbook is distributed to all member schools. It warns that concussions are treated too lightly, and recommends that concussed players receive rest and constant supervision, and not be allowed to play or practice until symptoms have been gone for 48 hours. For symptoms lasting longer than 48 hours, it recommends players "not be permitted to compete for 21 days or longer, if at all."

1937: At its annual meeting, the American Football Coaches Association declares that concussed players should immediately be taken out of a game. "Sports demanding personal contact should be eliminated after an individual has suffered a concussion."

1973: The condition later named Second Impact Syndrome is first identified. It occurs when an athlete receives a concussion while still suffering the effects of a previous one, and according to a 2013 study in the Journal of Neurosurgery it carries a 90 percent mortality rate. "Those who do survive second impact syndrome are neurologically devastated," reports the director of the Sports Concussion Clinic at Children's Hospital Boston.

1991: The Colorado Medical Society publishes a grading system for concussion severity and establishes strict guidelines for allowing players back into the game. It is quickly incorporated by the NCAA and high school football.

1994: The NFL acknowledges the danger of concussions for the first time, forming the Mild Traumatic Brain Injury Committee. It is co-chaired by Elliot Pellman, a rheumatologist who claimed to have a degree from Stony Brook. (He didn't. He attended medical school in Guadalajara, Mexico.) Pellman is the Jets' team doctor. He's also commissioner Paul Tagliabue's personal doctor.

Pellman reportedly tells one doctor on his team, "Don't talk to the press." He also tells Sports Illustrated, "Concussions are part of the profession, an occupational risk" and says a football player is "like a steelworker who goes up 100 stories, or a soldier."

1995: Pellman tries to speed up Boomer Esiason's return from a concussion. He uses an unproven system that involves the QB sitting in front of a computer screen and concentrating. Says Pellman:

"Imagine the equivalent of having a head filled with marbles knocked around after a hit. The biofeedback is trying to put them back in the same order. But we haven't had control studies to show whether the improvement is measurable."

1997: The American Academy of Neurology publishes its own guidelines for players returning to action after being concussed. It recommends removing players knocked unconscious from a game. The NFL later rejects the guidelines, with one of its consultants saying, "We see people all the time that get knocked out briefly and have no symptoms."

1999: The NFL's retirement board quietly begins giving out millions in disability payments to former players suffering cognitive decline, finding that they had become "totally disabled" due to "league football activities."

2000: A study presented at the American Academy of Neurology's annual meeting finds that 61 percent of former NFL players sustained concussions, with 79 percent of those injured saying they had not been forced to leave the game. Furthermore:

49% of the former players had numbness or tingling; 28% had neck or cervical spine arthritis; 31% had difficulty with memory; 16% were unable to dress themselves; and 11% were unable to feed themselves;

2002: Dr. Bennet Omalu examines the brain of Mike Webster and sees a splotchy accumulation of tau protein, evidence of a brain disease that Omalu calls Chronic Traumatic Encephalopathy. CTE is a neurological degenerative disease most often found in the brains of boxers, and provides a direct link between head trauma and dementia later in life. (A 2013 paper in the Journal of the International Neuropsychological Society will dispute that CTE is a unique disease.)

2003: A study of retired football players finds that having multiple concussions doubled their risk of developing depression later in life.

Meanwhile, the MTBI committee releases the first results of its study. It finds that concussions have no long-term health effects.

Wayne Chrebet is concussed during a game and examined by Pellman, the Jets physician and MTBI committee co-chair. Pellman reportedly tells Chrebet, "This is a very important for your career" and sends him back into the game. Chrebet's symptoms persist after the game, and he is placed on season-ending injured reserve.

2005: The MTBI committee releases more findings. Among the conclusions: "Return to play does not involve a significant risk of a second injury either in the same game or during the season."

A study by the UNC Center for the Study of Retired Athletes finds a connection between concussions and Alzheimer's disease, dementia, and depression in former NFL players. More, it finds a correlation between the long-term effects on the number of concussions the player suffered.

2005: A survey of retired NFLers finds a history of concussions makes a player five times as likely to suffer cognitive impairment.

2006: ESPN discontinues its "Jacked Up!" segment highlighting the hardest and most spectacular hits of the weekend's games.

2007: The UNC Center for the Study of Retired Athletes publishes a study linking concussions and depression in former NFL players. One member of the MTBI committee, a consultant for the Colts, calls the study "virtually worthless."

2007: Dr. Ira Casson, co-chairman of the MTBI, says in an interview on HBO Real Sports that there is no link between head injuries and depression, dementia, early onset Alzheimer's, or "any long term problems."

A pamphlet is distributed to the players and reads in part, "Current research with professional athleteshas not shown that having more than one or two concussions leads to permanent problems if each injury is managed properly."

2008: An NFL-commissioned survey finds former players suffer Alzheimer's and dementia at a rate 19 times higher than for non-players between the ages of 30-49. The NFL calls the study inconclusive.

2009: For the first time, the NFL acknowledges the effects of head trauma. League spokesman Greg Aiello says, "It's quite obvious from the medical research that's been done that concussions can lead to long-term problems."

The first lawsuits against the league are filed. Over the next few years, they will balloon to nearly 250 cases and 5,000 plaintiffs, including former players from the 1940s.

2009: Chris Henry dies after either falling or jumping from a moving truck. His mother says he had been having headaches and mood swings. He is later diagnosed with CTE.

2010: Casson appears before Congress. He says CTE "has never been linked to athletics or head trauma."

"We all had issues with some of the methodologies described, the inherent conflict of interest that was there in many areas, that was not acceptable by any modern standards or not acceptable to us. I wouldn’t put up with that, our universities wouldn’t put up with that, and we don’t want our professional reputations damaged by conflicts that were put upon us."

The NFL puts up posters in every locker room warning players of the effects of concussions, and announces penalties and fines for tackles that target the head.